The US President has been condemned by critics over his repeated refusal to accept uncomfortable facts, including his denial that Hillary Clinton won the popular vote during the 2016 presidential election.

But the 71-year-old has begun to fixate on his most notorious untruth in recent months; that former president Barack Obama’s birth certificate is fake, reports The New York Times.

The “birther” conspiracy theory was first peddled during the 2008 presidential race and asserts Mr Obama was ineligible to be president because he was not born a citizen of the US.

It is widely considered a racist reaction to Mr Obama’s status as the first African-American to become president of the country.

Despite appearing to drop the claims after taking office, Mr Trump has reportedly used private conversations to question the authenticity of his predecessor’s birth certificate in recent months.

Donald Trump's least presidential moments so far...

The billionaire has also never accepted he lost the popular vote to Ms Clinton, resorting to the false claim that there were millions of people who voted illegally. He has made this claim repeatedly in recent months, the newspaper alleges.

Jeff Flake, a Republican Senator and critic of the President, said Mr Trump’s disregard for “shared facts” is “dangerous to democracy”.

“On so many of these, there’s empirical evidence that says no: You didn’t win the popular vote, there weren't more people at your inauguration than ever, that was your voice on that tape, you admitted it before,” he said.

After images showed far more people attended Capitol Hill to celebrate Mr Obama’s 2009 inauguration than went to his, the President accused the media of lying to "demean” him.